Effect of electrochemically modified titanium-zirconium (TiZr) alloy implant surface on osseointegration: an in vitro and in vivo study

Abstract

Titanium (Ti) and its alloys have been used as biomaterial for dental implants for decades. Recently, titanium-zirconium (TiZr) alloy has been developed as an alternative implant material with higher mechanical strength, superior corrosion resistance and biocompatibility. The published data for commercially-available oral implants fabricated from titanium-zirconium alloy (Roxolid, Straumann etc) involves surface modification by modified sandblasting and large-grit acid etching in a low oxygen & carbon environment (SLActive®). An alternative surface modification involves anodization, which forms a thick oxide layer on titanium that is reported to enhance the bone formation around implants. Therefore, the purpose of this thesis is two-fold. Firstly, the aim was to develop and optimise a methodology to electrochemically (anodization) modify TiZr and study its surface characteristics and cytocompatibility by cell culture experiments. Secondly, the aim was to assess the effect of anodized TiZr (aTiZr) surface on osseointegration by an in vivo animal study. Titanium (Ti) and titanium-zirconium (TiZr) discs with smooth, polished surface were anodized in an electrolyte containing DL-α-glycerophosphate disodium salt hydrate (DL-α-GP) and calcium acetate (CA) at 60mA/cm2 and 300V. This is the first study attempting to anodize TiZr using this technique. The surface characteristics were analyzed for elemental composition by electron dispersive spectroscopy (EDS) and phase composition by X-ray diffraction (XRD). The surface micro topography of the anodized discs was characterized by using scanning electron microscopy (SEM). Surface roughness was analyzed using atomic force microscopy (AFM) and contact angle calculated using goniometry. The human osteosarcoma cell line SaOS-2 was used to assess cell behavior on the modified surfaces. Cell culture experiments such as cell viability, proliferation, differentiation and mineralization were carried out to assess their bioactivity. The results of the in vitro experiments indicated that aTiZr exhibited optimum surface characteristics with a rough, micro-nano porous and hydrophilic surface. The cytocompatibility of the modified surface was verified by cell culture experiments with enhanced osteogenic effects and mineral deposits on aTiZr equivalent to aTi. To assess biocompatibility in an intraosseous environment, the anodized TiZr discs were implanted in sheep femur epicondyles. A novel animal model design with custom-made surgical template was developed to study percentage of bone to implant contact and new bone fill simultaneously. The results indicated that aTiZr was biocompatible with significantly higher percentages of bone implant contact and new bone fill compared to unmodified TiZr surfaces but similar to aTi. In summary, Ti-Zr alloy with low modulus and high strength when electrochemically modified, formed a nano-to-microporous hydrophyllic surface that enhanced osteoblast cell behavior and influenced early osseointegration equivalent to anodized pure titanium.